Rails has_many through association not updating join model - ruby-on-rails-4

Question

I have the following models:
class Project < ActiveRecord::Base
has_many :memberships
has_many :users, through: :memberships
end
class User < ActiveRecord::Base
has_many :memberships
has_many :projects, through: :memberships
end
class Membership < ActiveRecord::Base
belongs_to :user
belongs_to :project
end
and the migration for memberships
def change
create_table :memberships do |t|
t.belongs_to :user
t.belongs_to :project
t.boolean :admin
end
end
In my controller, I want to create a project for the current user
def create
#project = current_user.projects.build(project_params)
if #project.save
render json: #project, status: :ok
else
render json: { errors: #project.errors }, status: :unprocessable_entity
end
end
def project_params
params.require(:project).permit(:name)
end
When #project.save is called, the SQL in the log:
INSERT INTO "projects" ("created_at", "name", "updated_at") VALUES (?, ?, ?) [["created_at", "2014-12-09 17:18:55.298566"], ["name", "Test"], ["updated_at", "2014-12-09 17:18:55.298566"]]
So memberships is never updated, so the project gets made but it is not associated with the current user. I think rails is supposed to do that automatically when using the relationship to build the object, but that doesn't seem to be the case here.
It will work if I do
#project = current_user.projects.build(project_params)
#project.users << current_user
But that feels incorrect. Where am I going wrong?